Controlling warping in integrated circuit devices

ABSTRACT

Techniques for integrated circuit device fabrication are provided. In one aspect, an integrated circuit device comprises a base, at least one die attached to the base, and a counterbalancing layer on at least a portion of at least one side of the base adapted to compensate for at least a portion of a thermal expansion difference existing between the base and the die. In another aspect, warping of an integrated circuit device comprising at least one die attached to a base is controlled by applying a counterbalancing layer to at least a portion of at least one side of the base adapted to compensate for at least a portion of a thermal expansion difference existing between the base and the die.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/163,453, filed Jun. 27, 2008, which is a divisional of U.S. patentapplication Ser. No. 11/095,929, filed Mar. 31, 2005, each incorporatedby reference herein.

FIELD OF THE INVENTION

The present invention relates generally to integrated circuit devices,such as power transistor devices, and more particularly to techniquesfor controlling warping in such devices.

BACKGROUND OF THE INVENTION

Metal oxide semiconductor (MOS) power transistor devices have gainedpopularity in certain applications for their high power capabilities.There are various types of MOS power transistor devices, includingdiffused metal oxide semiconductor (DMOS) devices, such as lateral DMOS(LDMOS) devices, vertical DMOS devices and trench DMOS devices.

A MOS power transistor device may comprise one or more transistor diesmounted on a base. The die(s), base and potentially other components ofthe transistor device may be contained, at least partially, in apackage. For example, the base is typically surrounded by polymer orceramic walls that form cavity sidewalls of the package. A polymer orceramic lid is then placed on top of the sidewalls. These “packaged”components are referred to herein collectively as a device package. Thebase, which typically comprises the bottom of the device package, may bemounted on a heat sink to remove heat from the device package duringoperation.

A method commonly used to mount the die on the base is soldering. Duringsoldering, however, elevated temperatures are experienced causing boththe die and the base to expand. The die and the base typically comprisedifferent materials, and therefore expand by different magnitudes. Forexample, dies typically comprise silicon which, when heated, will expandby a lesser magnitude than the base, which typically comprises copper,or some alloy thereof. This differential expansion and its associatedcontraction, e.g., during cooling, can cause warping of the base.

Warping of the base is undesirable. For example, depending on the amountof warping that occurs, the device package may not be suitable forattachment to the heat sink. Specifically, attaching a warped base to aheat sink can result in voids, or gaps, being present between the baseand the heat sink. These voids can serve as an impediment to the thermalpath between the device package and the heat sink, and thus can lead tooverheating of the device.

Therefore, techniques for minimizing or eliminating warping in powertransistor devices, and other types of integrated circuit devices, wouldbe desirable.

SUMMARY OF THE INVENTION

Techniques for integrated circuit device fabrication are provided. Inone aspect of the invention, an integrated circuit device comprises abase, at least one die attached to the base, and a counterbalancinglayer on at least a portion of at least one side of the base adapted tocompensate for at least a portion of a thermal expansion differenceexisting between the base and the die.

In another aspect of the invention, warping of an integrated circuitdevice comprising at least one die attached to a base is controlled byapplying a counterbalancing layer to at least a portion of at least oneside of the base adapted to compensate for at least a portion of athermal expansion difference existing between the base and the die.

In an illustrative embodiment, an integrated circuit device includes acounterbalancing layer comprising nickel (Ni) deposited on a side of abase opposite one or more silicon (Si) dies. The counterbalancing layercompensates for differential expansion and contraction between the baseand the Si dies by itself expanding and contracting in a manner thatcancels out at least a portion of the differential expansion andcontraction of the Si dies and the base.

Advantageously, the invention in the illustrative embodiment preventsmost, if not all, of the warping of the base, resulting in a flat orsubstantially flat surface for attachment of the integrated circuitdevice to a heat sink.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a conventional power transistor devicemounted on a heat sink;

FIG. 2 is a diagram illustrating an exemplary methodology forcontrolling warping of a power transistor device;

FIG. 3 is a diagram illustrating an exemplary power transistor devicehaving a segmented counterbalancing layer;

FIG. 4 is a diagram illustrating an exemplary power transistor devicehaving a hatched counterbalancing layer;

FIG. 5 is a diagram illustrating an exemplary power transistor device,having a counterbalancing layer, soldered to a heat sink;

FIG. 6 is a table displaying properties of several materials used inpower transistor devices;

FIG. 7 is a table displaying amounts of warping experienced by a basehaving either a nickel (Ni) layer present at various thicknesses, or noNi layer at all;

FIG. 8 is a graph illustrating warping of copper (Cu) alloy bases ofvarious thicknesses, each having a ten μm thick counterbalancing layercomprising Ni;

FIG. 9 is a graph illustrating warping of Cu alloy bases havingdifferent compositions and various thicknesses;

FIG. 10 is a graph illustrating warping of 54 mil thick Cu alloy baseshaving either counterbalancing layers of various thicknesses, or nocounterbalancing layer at all; and

FIG. 11 is a graph illustrating warping of 64 mil thick Cu alloy baseshaving either counterbalancing layers of various thicknesses, or nocounterbalancing layer at all.

DETAILED DESCRIPTION

Before describing the inventive aspects of the present invention, aconventional power transistor device is described. For example, as shownin FIG. 1, a conventional power transistor device comprises silicon (Si)dies 102 and 104 mounted on, e.g., by soldering, base 106, whichtypically comprises a metal. The power transistor device is solderedonto metal heat sink 108, e.g., at solder regions 110 and 112.

Because silicon dies 102 and 104 have a different composition (e.g.,comprise different materials) from base 106 (and potentially from eachother) they will have different coefficients of thermal expansion (CTE)and thus will expand a different amount than base 106 when exposed tothe heat generated during solder attachment to base 106. Thisdifferential thermal expansion and its associated contraction (e.g.,during cooling) will cause base 106 to warp. Specifically, to liquefythe solder, depending on the solder composition, temperatures as high asabout 400° C. may be employed. These elevated temperatures cause silicondies 102 and 104, and base 106 to expand. Once the solder melts, thetemperature is then reduced to allow the solder to re-solidify and forma bond. Typical gold/tin (Au/Sn) solders begin to re-solidify around320° C. Thus, at this temperature, Si dies 102 and 104 become fixed tobase 106 (e.g., by the re-solidified solder). Also, at this temperature,Si dies 102 and 104 and base 106 have expanded by different amounts,however, their positions, relative to one another, are now fixed by thesolidified solder. As the device further continues to cool, e.g., toroom temperature, Si dies 102 and 104, and base 106, will begin tocontract. Since, Si dies 102 and 104, and base 106 will contract by anamount proportional to their expansion, Si dies 102 and 104 and base 106will contract by different amounts. With their positions relative to oneanother being fixed, this differential contraction can cause base 106 towarp, as shown.

Attaching a warped power transistor device to a heat sink, for exampleas shown in FIG. 1, will result in void(s), e.g., void 114, beingformed. These voids trap air which acts as an impediment to thermalflow. Specifically, the thermal path, e.g., illustrated by arrows 116,will be constrained to areas where the solder forms a continuous bondbetween base 106 and metal heat sink 108 and insufficient cooling mayresult. Insufficient cooling can lead to overheating of the powertransistor device.

In FIG. 2, exemplary methodology 200 for controlling warping of a powertransistor device is presented. In step 202, base 208 is provided. Base208 may comprise any suitable materials, including, but not limited to,metals, such as copper (Cu) and alloys thereof, for example, Cu/tungsten(Cu/W) alloys, and non-metals, such as silicon carbide (SiC), aluminumnitride (AlN) and chemical vapor deposited (CVD) diamond. According toan exemplary embodiment, as will be described in detail below, base 208may have a thickness of about 50 mils to about 70 mils.

According to an exemplary embodiment, base 208 is produced from a sheetusing standard processing techniques. For example, base 208 may bestamped from a large Cu or Cu alloy sheet. Such a stamping process wouldtypically result in the base produced being flat or substantially flat,e.g., any deviations from planarity being less than or equal to aboutone mil. For example, when placed on a flat surface, any raise of theedges from the flat surface would be less than or equal to about onemil.

In step 204, counterbalancing layer 210 is deposited on one side of base208. Counterbalancing layer 210 acts as a warp compensating layer, e.g.,compensating the differential expansion and contraction between base 208and dies 212 and 214, see step 206, below, which, if left uncompensated,may result in warping of base 208.

Counterbalancing layer 210 may comprise any suitable material,including, but not limited to, metals, such as nickel (Ni), molybdenum(Mo), titanium (Ti), chromium (Cr), tungsten (W) and combinationscomprising at least one of the foregoing metals. By way of example only,counterbalancing layer 210 may be deposited on base 208 using anysuitable deposition techniques, including, but not limited to,electrochemical plating techniques, CVD, sputtering, electron-beam(e-beam) deposition and plasma enhanced deposition. Further, as shown inFIG. 2, counterbalancing layer 210 is preferably deposited on a singleside of base 208. Also, techniques other than deposition may be used toform a counterbalancing layer in accordance with the invention.

Counterbalancing layer 210 compensates for the differential expansionand contraction between base 208 and dies 212 and 214 by itselfexpanding and contracting in a manner that cancels out at least aportion of the differential expansion and contraction of the dies andbase. For example, according to an exemplary embodiment,counterbalancing layer 210 is in a fixed position relative to base 208and opposite the dies (which are also in a fixed position relative tobase 208, see description of step 206, below). Counterbalancing layer210 will thus heat up, e.g., during soldering of the dies to base 208,and expand and subsequently contract concurrently with base 208 and thedies. The coordinated expansion and contraction of these interrelatedlayers can act to prevent warping of the device. The relative expansionof some exemplary materials will be described in detail below. As willalso be described in detail below, the thickness and configuration ofthe counterbalancing layer may be varied to tune its compensatingproperties.

According to an exemplary embodiment, counterbalancing layer 210 is acontinuous layer, extending over a substantial portion of at least onesurface of base 208. Counterbalancing layer 210 does not, however, haveto be continuous. By way of example only, counterbalancing layer 210 maycomprise a segmented layer, e.g., having perforations or other openingsor indentations extending through at least a portion of the thickness ofthe layer. According to an exemplary embodiment, counterbalancing layer210 comprises a segmented layer having indentations on one or moresurfaces thereof. According to another exemplary embodiment,counterbalancing layer 210 comprises a segmented layer with perforationsextending through the entire thickness of the layer, the length of eachsegment running parallel with the direction of warping. See FIG. 3,below. Also, counterbalancing layer 210 may comprise a hatched layer.See FIG. 4, below.

Further, as will be described, for example, in conjunction with thedescription of FIG. 6, below, the thickness of counterbalancing layer210 may be varied to tune its compensating properties.

In step 206, dies 212 and 214 are soldered onto a side of base 208opposite counterbalancing layer 210. As shown in FIG. 2, dies 212 and214 are attached to a side of base 208 opposite counterbalancing layer210.

Dies 212 and 214 each may comprise any suitable materials, including,but not limited to, one or more of Si, indium phosphide (InP), indiumgallium phosphide (InGaP), indium gallium arsenide phosphide (InGaAsP),gallium arsenide (GaAs), silicon carbide (SiC), silica germanium (SiGe),indium tin oxide (InSnO), black diamond, crystallized carbon, polymericsemiconductor and lithium niobate (LiNbO₃). Further, dies 212 and 214may comprise the same, or different, materials from each other. In oneexemplary embodiment, both dies 212 and 214 comprise Si.

As highlighted above, counterbalancing layer 210 may be segmented. InFIG. 3, power transistor device 300 having a segmented counterbalancinglayer is shown. Power transistor device 300 comprises base 302 havingdie 312 on a side thereof. Base 302 also has counterbalancing layersegments 304 and 306 (having perforation 308 therebetween) on a sideopposite die 312.

According to the configuration shown in FIG. 3, counterbalancing layersegments 304 and 306 run in the same direction as, e.g., are parallelto, the direction of the warping of base 302. It is important to notethat the warping of base 302 is shown exaggerated, primarily toillustrate an exemplary direction of warping 310. However, according tothe teachings presented herein, some, or all, of the warping of base 302will be prevented by counterbalancing layer segments 304 and 306, andthus would be barely, if at all, visible.

It is also to be understood that depending on the configuration of thepower transistor device, warping may occur in more than one direction.By way of example only, if the power transistor device is configured tohave one side longer than the other, e.g., making it long and narrow,then warping may occur predominately in one direction, for example, asshown in FIG. 3. However, if the power transistor device is configuredto have sides that are equal, or near equal, in length to each other,then warping may occur in more than one direction, for example, as shownin FIG. 4.

In FIG. 4, power transistor device 400 is configured to have sides thatare equal, or near equal, in length to each other. As a result, warpingoccurs in more than one direction, e.g., direction of warping 402 anddirection of warping 404. In FIG. 4, power transistor device 400comprises base 406 having hatched counterbalancing layer 408 on a sidethereof, e.g., on a side opposite one or more dies (not visible from theangle shown). Hatched counterbalancing layer 408, while not a continuouslayer, can serve to counterbalance both directions of warping 402 and404.

It should be noted that the counterbalancing layer configurations shownin FIGS. 3 and 4 are merely exemplary, and the teachings herein shouldnot be limited to these, or any other, particular configurations.

As above, the warping of base 406 is shown exaggerated, primarily toillustrate exemplary directions of warping 402 and 404. However,according to the teachings presented herein, some, or all, of thewarping of base 406 will be prevented by hatched counterbalancing layer408, and thus would be barely, if at all, visible.

In FIG. 5, a power transistor device 500 having a counterbalancing layer508 is shown soldered to a heat sink, e.g., heat sink 510. The powertransistor device 500, similar to the power transistor device 200described in conjunction with the description of FIG. 2, above,comprises base 506 having Si dies 502 and 504 mounted thereon andcounterbalancing layer 508 on a side of base 506 opposite the dies. Thepower transistor device is attached to metal heat sink 510 by solderlayer 512. It is important to note from FIG. 5 that, since base 506 isflat, a continuous bond via solder layer 512 is formed with heat sink510. As such, no voids are present. Therefore, an uninterrupted thermalpath, e.g., as indicated by arrows 514, between silicon dies 502 and504, and heat sink 510 is created.

FIG. 6 is a table displaying coefficient of thermal expansion (CTE),modulus of elasticity (modulus) and thickness values for Si, e.g., diematerial, Cu, e.g., base material, and Ni, e.g., counterbalancing layermaterial.

It should be noted from the table, that the CTE for Cu, e.g., 16.9 partsper million per ° C. (ppm/° C.), is greater than five times that for Si,e.g., three ppm/° C. This expansion differential is what, when notcounteracted, causes warping of the device. The CTE for Ni, e.g., 13.1ppm/° C., however, is on the same order of magnitude, but less than thatof the Cu. As a result, the Ni layer will expand an amount similar to,yet less than, the Cu. The differential expansion (and subsequentcontraction) of each of these three interrelated layers, at least inpart, cancel each other out, and maintain the base in a flat, orsubstantially flat state.

The modulus, or modulus of elasticity, values define the ratio of stressto strain for each of the materials (measured in gigapascals (GPa)). Themodulus of elasticity is basically an index of the “stiffness” of amaterial, e.g., how easily does it bend. Modulus of elasticity valuesmay be obtained, for example, from materials suppliers, as will beappreciated by those skilled in the art.

In the table of FIG. 6, the CTE and modulus of elasticity data arepresented for certain thicknesses of the three materials. For example,data is given for Si at a thickness of two mils, for Cu at a thicknessof 50 mils and for Ni at a thickness of 2.5 micrometers (μm) to 8 μm.

FIG. 7 is a table displaying tip deflection values (in mils) andreduction values of a base having a continuous Ni layer present atvarious thicknesses, or no Ni layer present at all. Thickness values arepresented both in μm and in micro-inches (μ-inches).

The tip deflection values provide a measure of deviations from planarityof the base and can thus be used to represent the magnitude of thedeformation experienced in the base. The reduction (e.g., reduction inwarping) values provide a measure of the tip deflection value for a basehaving a Ni layer of a particular thickness as compared to a basewithout any Ni layer. For example, by applying a Ni layer having athickness of 50 μm to a base, an 82.6 percent reduction in warping, ascompared to a similar base without a Ni layer, is experienced (e.g., areduction value of −82.6 percent). Therefore, the reduction valueprovides a good representation of the magnitude of warping prevented bythe Ni layer.

It should be noted that the addition of a Ni layer having certainthicknesses can cause the base to warp in a direction opposite to thatexperienced with no Ni layer. For example, when a continuous 100 μm Nilayer is applied to the base, a reduction value of −159.5 percent isobtained. Therefore, if for example, the base without a Ni layer warpsto be concave, e.g., relative to a given flat surface, then the additionof a 100 μm Ni layer would render it convex relative to the same flatsurface. This situation is further illustrated in FIG. 11.

The graph in FIG. 8 illustrates warping of Cu alloy bases of variousthicknesses, each having a ten μm thick counterbalancing layercomprising Ni. The Cu alloy used, C151, comprises about 99.9 percent byweight Cu and about 0.1 percent by weight zirconium (Zr), based on thetotal weight of the alloy (referred to hereinafter as “C151”). Cu alloyC110, comprising greater than or equal to about 99.9 percent by weightCu and less than or equal to about 0.05 percent oxygen (O), based on thetotal weight of the alloy (referred to hereinafter as “C110”), isincluded for comparison purposes.

In the graph, warping is measured as a function of the distance from thecenter of the package, e.g., from the center of the base. The graphillustrates that the thicker the base employed, the less the amount ofwarping that occurs. Therefore, some amount of warping may be preventedsimply by varying the thickness of the base. However, the thickness ofthe base is constrained by the desired overall height of the device,which is typically required to be suitable for integration into smallspaces. Therefore, the overall height of the device is generally notsufficient to accommodate a base thick enough to, on its own,substantially eliminate warping of the device.

Further, the C110 base exhibits a greater amount of warping, as comparedto the C151 base of the same thickness. Therefore, some amount ofwarping may also be prevented by selecting different base compositions.See, for example, FIG. 9, below.

As the heading of the graph in FIG. 8 indicates, the ten μm Ni layer onthe bottom of the base (for example, on a side of the base opposite,e.g., a Si die or Si dies) is considered an “extra” Ni layer, as a smallamount of Ni may be already present on the base. Specifically, smallamounts of Ni and Au are typically plated on the base to enhancesolderability (e.g., the Ni prevents, for example, Cu from the base fromdiffusing through the Au and affecting the solder). This amount ofnickel already plated on the base is negligible with regard tocontrolling warping of the device. Au is used to promote solder wettingand enhance solderability. The amount of Au already plated on the baseis likewise also negligible with regard to controlling warping of thedevice.

FIG. 9 is a graph illustrating the amount of warping experienced bydifferent Cu alloy bases of different thicknesses during solderattachment of a Si die. No counterbalancing layer is present in any ofthe samples.

As mentioned, for example, in conjunction with the description of FIG.8, above, the graph in FIG. 9 illustrates that some amount of warpingmay be prevented by selecting different base compositions. Specifically,the graph in FIG. 9 shows that when comparing Cu alloy bases of the samethickness, e.g., 64 mils, the base comprising C110 warps more than thebase comprising C151. Increasing the thickness of the base comprisingC151, e.g., to 74 mil, further decreases the amount of warpingexperienced.

FIG. 10 is a graph illustrating that when a continuous counterbalancinglayer comprising Ni is present, the thicker the counterbalancing layer,the less warping that occurs. The same base without a counterbalancinglayer is included for comparison.

The base comprises C151 which experiences a lesser amount of warping ascompared with a base of the same thickness comprising C110, as describedin conjunction with the description of FIG. 9, above. The base iscontained in a plastic cavity package. Warping of the base occurs as aresult of the process used to attach dies to the base, described above.

FIG. 11 is a graph illustrating that when a continuous counterbalancinglayer comprising Ni is present, the thicker the counterbalancing layer,the less warping that occurs. In fact, the graph illustrates that whenthe Ni layer has a thickness of 100 μm, as described above, the Cu alloybase will in fact warp inversely, as compared to a base without a Nilayer. The designation “Ni bot” indicates that the counterbalancinglayer comprising Ni is on the bottom of the Cu alloy base, e.g., on aside opposite the die(s).

As in FIG. 10, described above, the base comprises C151. Also asdescribed above, the base is contained in a plastic cavity package.Warping of the base occurs as a result of the process used to attach oneor more dies to the base.

In comparison with the 54 mil thick bases of FIG. 10 havingcounterbalancing layers of the same thickness, the 64 mil thick baseshaving a five μm and a ten μm Ni layer, experience less warping.However, as highlighted above, the thickness of the base is constrainedby the desired overall height of the device

Therefore, the addition of a counterbalancing layer can counteract atleast a portion of warping experienced in power transistor devicesoccurring as a result of heat being generated during certain attachmentprocesses, e.g., during soldering. Further, properties, e.g., thicknessand composition, of the counterbalancing layer, as well as, of thecomponents of the device, can be varied to adjust warp compensation.

Further, while the instant description highlights power transistordevices comprising particular materials and having particularconfigurations, it is to be understood that the present teachings may beapplied to a wide variety of other integrated circuit devices comprisingother materials and having other configurations. Specifically, thepresent techniques may be applied to any integrated circuit device thatexperiences differential thermal expansion and contraction. By way ofexample only, the present techniques would be applicable to anintegrated circuit device that experiences differential thermalexpansion and contraction as a result of heat generated duringoperation.

At least a portion of the techniques of the present invention describedherein may be implemented in an integrated circuit. In formingintegrated circuits, a plurality of identical die is typicallyfabricated in a repeated pattern on a surface of a semiconductor wafer.Each die includes a device described herein, and may include otherstructures or circuits. The individual die are cut or diced from thewafer, then packaged as an integrated circuit. One skilled in the artwould know how to dice wafers and package die to produce integratedcircuits. Integrated circuits so manufactured are considered part ofthis invention.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention as set forth in the appended claims.

1. A method of controlling warping of a power transistor methodcomprising at least one die attached to a base, the method comprisingthe step of: applying a counterbalancing layer to at least a portion ofat least one side of the base adapted to compensate for at least aportion of a thermal expansion difference existing between the base andthe die; and varying one or more of composition and thickness of thecounterbalancing layer to affect thermal expansion properties of thecounterbalancing layer.
 2. The method of claim 1, wherein the thermalexpansion difference occurs as a result of heat generated duringattachment of the die to the base.
 3. The method of claim 1, wherein thethermal expansion difference between the base and the die is due, atleast in part, to the base having a different coefficient of thermalexpansion from the die.
 4. The method of claim 1, wherein the basecomprises one or more of metals and non-metals.
 5. The method of claim1, wherein the base comprises one or more of copper and alloys thereof.6. The method of claim 1, wherein the base comprises one or more ofsilicon carbide, aluminum nitride and chemical vapor deposited diamond.7. The method of claim 1, wherein the counterbalancing layer comprisesone or more of nickel and gold.
 8. The method of claim 1, wherein thecounterbalancing layer is segmented, having indentations on at least onesurface thereof.
 9. The method of claim 1, wherein the counterbalancinglayer is segmented, having perforations extending through the entirethickness of the layer, the length of each segment running parallel witha direction of warping.
 10. The method of claim 1, wherein thecounterbalancing layer comprises a hatched layer.
 11. The method ofclaim 1, wherein the counterbalancing layer is applied as a continuouslayer extending substantially over at least one side of the base. 12.The method of claim 1, wherein the counterbalancing layer is applied toa side of the base opposite the die.
 13. A method of controlling warpingof a power transistor method comprising at least one die attached to abase, the method comprising the step of: applying a counterbalancinglayer to at least a portion of at least one side of the base adapted tocompensate for at least a portion of a thermal expansion differenceexisting between the base and the die; and varying one or more ofcomposition and thickness of the base to affect thermal expansionproperties of the base.
 14. The method of claim 13, wherein the thermalexpansion difference occurs as a result of heat generated duringattachment of the die to the base.
 15. The method of claim 13, whereinthe thermal expansion difference between the base and the die is due, atleast in part, to the base having a different coefficient of thermalexpansion from the die.
 16. The method of claim 13, wherein the basecomprises one or more of metals and non-metals.
 17. The method of claim13, wherein the base comprises one or more of copper and alloys thereof.18. The method of claim 13, wherein the base comprises one or more ofsilicon carbide, aluminum nitride and chemical vapor deposited diamond.19. The method of claim 13, wherein the counterbalancing layer comprisesone or more of nickel and gold.
 20. The method of claim 13, wherein thecounterbalancing layer is segmented, having indentations on at least onesurface thereof.
 21. The method of claim 13, wherein thecounterbalancing layer is segmented, having perforations extendingthrough the entire thickness of the layer, the length of each segmentrunning parallel with a direction of warping.
 22. The method of claim13, wherein the counterbalancing layer comprises a hatched layer. 23.The method of claim 13, wherein the counterbalancing layer is applied asa continuous layer extending substantially over at least one side of thebase.
 24. The method of claim 13, wherein the counterbalancing layer isapplied to a side of the base opposite the die.